The “mixed raster content” (MRC) representation of documents provides the ability to represent color images and either color or monochrome text. In an MRC representation, an image is represented by more than one image plane. The main advantage of the MRC representation of documents is to provide an efficient way to store, transmit, and manipulate large digital color documents. The method exploits the properties of the human vision system, where the ability to distinguish small color variations is greatly reduced in the presence of high-contrast edges. The edge information is normally separated from the smoothly varying color information, and encoded (possibly at higher spatial resolution than 1 bit per pixel) in one of the planes, called the Selector plane. Following a careful separation, the various planes could be independently compressed using standard compression schemes (such as JPEG and G4) with good compression and high quality at the same time.
FIG. 1 shows the general MRC representation. The representation typically comprises three independent planes: foreground, background, and a selector plane. (In some embodiments, other planes, such as including “rendering hints,” are also provided.) The background plane is typically used for storing continuous-tone information such as pictures and/or smoothly varying background colors. The selector plane normally holds the image of text (binary) as well as other edge information (e.g., line art drawings). The foreground plane usually holds the color of the corresponding text and/or line art. However, the MRC representation only specifies the planes and their associated compression methods: it does not otherwise restrict nor enforce the content of each of the planes. The content of each of the planes may be defined appropriately by an implementation of the MRC representation.
The foreground and background planes are defined to be two full-color (L, a, b) or YCC planes. The Selector plane is defined as a binary (1-bit deep) plane. One exemplary MRC representation specifies that the foreground and background are to be JPEG compressed, or more generally, can be a suitable image for a lossy compression technique, and that the selector plane is to be ITU-G4 compressed, or more broadly is suitable for a lossless compression technique. In general, the foreground, background, and selector planes can all have different spatial resolutions, and they are not required to maintain the original source input resolution.
The method for assembling back a “segmented” MRC image from its components (i.e., planes) is by “pouring” the foreground colors through the selector plane “mask” on top of the background plane, thus overwriting the previous content of the background plane at these locations. In other words, the assembly is achieved by multiplexing between the foreground and background information on a pixel by pixel basis, based on the binary control signal of the selector plane. For example, if the selector value is 1 or ON, the content of foreground is used; otherwise (i.e., for selector value=0 of OFF) the content of background is used. The multiplexing operation is repeated on a pixel by pixel basis until all of the output pixels have been defined.
The main advantage of the MRC representation of documents is to provide an efficient way to store, transmit, and manipulate large digital color documents. The method exploits the properties of the human vision system, where the ability to distinguish small color variations is greatly reduced in the presence of high-contrast edges. The edge information is normally separated from the smoothly varying color information, and encoded (possibly at higher resolution than one selector sample per source pixel) in the selector plane. Following a careful separation, the various planes could be independently compressed using standard compression schemes (such as JPEG and G4) with good compression and high quality at the same time.
The present disclosure relates to improving the quality of images which are rendered and compressed in MRC, especially in avoiding artifacts called “fringe effects,” which result from imperfections in selecting a compression technique (lossy or lossless) near a boundary in the image between foreground and background portions.